Yeah but for sci-fi that would be boring
Well possibility is that we are the oldest sentient species, anyone ever consider that? No....why? Because it just isn't something we get in our puny little heads (mainly I think it started as a way to shut out the Creationist religions who say man came first so Evolutionists just said LIKE HELL) but anyway seriously even from my opinion (I'm a christian so I tend to think with the bible here lol) even if we acknowledge evolution as a creation factor.
Why is it not possible that we are the oldest race yet to acheive sentient status? I mean if you look at evolution it is
supposed to have taken our ancestors a few hundred million years to acheive sentient status so maybe other planets got started a little after us
Also possible: There ARE other expansionist races out there they just might not be in our block of the galaxy, so maybe if and when we get out there WE will be the expansionist Empire crushing all in our wake.
I am not joking here either btw.